Ores mined for their gold and/or silver content typically contain other metals, such as mercury and a variety of base metals. The lower the grade of the ore, the more likely it is to contain metal impurities which should be removed before the gold and/or silver is refined. Of all the metals likely to be found in gold and/or silver-containing ores, it is the mercury in particular which is the most difficult to remove from cyanide solution and presents the most serious threat of contamination.
In one method widely used to process gold-containing ores, the ore is contacted with a cyanide solution which converts the gold in the ore to soluble gold cyanide. Other precious and base metals in the ore also go into solution, primarily as metal cyanide complexes. The gold is then recovered by one of several well known processes, such as Merrill-Crowe precipitation. Typically, some of the other precious and base metals in solution with the gold cyanide will precipitate out with the gold. The removal of these other metals, especially the mercury, from the Merrill-Crowe precipitate is essential to the successful refining of the gold.
One procedure currently used to separate mercury from the precious metal precipitate formed during Merrill-Crowe precipitation subjects the precipitate to a temperature of about 650.degree. C. (1200.degree. F.) in an electrically heated retort for about 24 hours. The mercury and some zinc are subsequently recovered in a pipe condenser. Although this method successfully removes the mercury, it is accompanied by serious disadvantages, not the least of which involves the hazards to the health of operating personnel posed by the highly toxic mercury vapors given off during the retorting process. The personnel which operate the special equipment needed for this process must be monitored constantly for signs of mercury poisoning. In addition, the large amount of energy necessary to maintain the precipitate at such extremely high temperatures for the lengthy time period required to remove the mercury makes this process an extremely costly one in many respects. Moreover, mercury is not always the only metal removed by the aforementioned process; zinc is also usually recovered and tends to accumulate in undesirable amounts in the condenser section of the equipment used to practice this method.